Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
While traditional metallocenes commonly include one or more cyclopentadienyl groups, many other ligands have been used. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. "Constrained geometry" or "open architecture" catalysts have been described (see, e.g., U.S. Pat. No. 5,624,878). Bridging ligands in these catalysts lock in a single, well-defined active site for olefin complexation and chain growth. Other bridged complexes are stereospecific catalysts for .alpha.-olefin polymerizations, providing a route to isotactic or syndiotactic polypropylene (see, for example, Herzog et al., J. Am. Chem. Soc. 118 (1996) 11988 and Mansel et al., J. Organometal. Chem. 512 (1996) 225).
Other known single-site catalysts replace cyclopentadienyl groups with one or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl, indolyl, (U.S. Pat. No. 5,539,124), or azaborolinyl groups (U.S. Pat. No. 5,902,866).
Substituted metallocenes, constrained-geometry catalysts, bridged complexes, and many heterometallocenes offer interesting advantages, including higher activity, control over polyolefin properties, and stereoregular polymers. Variety, however, comes at a price: ligands used to make many of these catalysts require costly multi-step syntheses from expensive and often hard-to-handle starting materials and reagents.
In sum, there is a continuing need for single-site catalysts that can be prepared inexpensively and in short order. In particular, there is a need for catalysts that can be tailored to have good activities and to give polyolefins with desirable physical properties.